It is known that porous expanded polytetrafluoroethylene (ePTFE) as is taught in U.S. Pat. No. 3,543,566 has many uses, such as breathable, water repellent membranes, filters, electrical insulation and the like. It is also well known that the properties of plastics can, in general, be modified by addition of various filler materials. Fluoropolymer materials such as ePTFE and the like are attractive candidates for electrical packaging applications because of their low dielectric constant, low moisture pickup and excellent thermal resistance; however, they exhibit poor dimensional stability and very high coefficient of thermal expansion (CTE). As such, it is desirable to modify the fluoropolymer substrate to achieve suitable properties. One such way is to add microfiber reinforcement into the ePTFE structure.
However, it has been difficult to make thin films of micro-fiber reinforced ePTFE because as the material thickness is reduced the filler/fiber particulate creates pinholes and tears. Furthermore, the presence of reinforcement fiber, particularly at levels above 10% (v/v) and especially micro-fiber, makes mixing and paste extrusion processing of these composites very difficult. It would be desirable to make very thin films of ePTFE which are free of pinholes and also have high strength and fiber reinforcement.
It is well known that inclusion of fiber can provide many desirable attributes to a composite such as decreased shrinkage, reduced creep, reduced coefficient of thermal expansion, etc. (Enclyclopedia of Polymer Science, McGraw Hill). Liquid crystal polymers (LCPs) show promise as a candidate to be used as fiber reinforcement due to their high modulus, low coefficient of thermal expansion (CTE) and low moisture absorbtion. In situ formation of LCP micro-fiber reinforcement in a polymer alloy so as to reinforce the molded or cast article by forming LCP fibrils is also described in the literature, in fact, commercially available thermoplastics have been blended and reinforced with liquid crystal polymers. (See, for example, S. L. Kwolek, et al., "Encyclopedia of Polymer Science, 2nd Ed., 9, p. 1. See, also, G. Crevecoeur and G. Broeninchx, Bull, Soc. Chim. Belg., 99 (11-12), 1990, p. 103). Composites of LCP with ePTFE have not been made heretofore because ePTFE is not melt processible, i.e, it is not readily processed in a thermoplastic fashion. It would be extremely useful to combine the desirable properties of ePTFE with the reinforcement of LCP.
Finally, thin films of liquid crystal polymers are taught in U.S. Pat. No. 4,325,903 to Wissbrun, et al. who extruded fiber of molded article of LCP by first heating, then cooling to just near the melting temperature of the LCP. L. P. Thomas and D. D. Roth (Chemtech, 20, (1990)p. 546)describe both anisotropic and balanced films of LCP; and in U.S. Pat. No. 4,333,907 to Urasaki, et al. is taught a melt-molded film entirely of LCP and a process of stretching a coherent film at T&gt;Tg. However, it is difficult to process LCPs--undesirable anisotropic properties are very difficult to eliminate in a film, especially an extruded film. The undesirable anisotropy leaves the film very notch-sensitive and typically produces a thin skin on the surface of the film, which is easy to peel away. The anisotropy is commonly reduced by increasing the amount of non-reactive filler into the LCP domain. (Foyer, P. D. Polymer Composites, 8(6) (1987) p. 379.
A flexible printed wiring board (PWB) substrate consisting of a film of LCP and/or a two phase interpenetrating network (IPN) with another thermoplastic is taught in U.S. Pat. No. 4,975,312 to Lusignea, et al. Here a substrate of a coherent, non-porous film having a tailored coefficient of thermal expansion (CTE) in the X-Y plane and thickness of less than 4 mil was produced by means of a counter rotating circular annular die method of extruding the LCP blends with other thermoplastics. The rotation, of the counter-rotating mandrels creates transverse shear flows that superimpose on the axial shear developed as the polymer melt is extruded through the die--the result is biaxial orientation. However, films made in such a manner are all non-porous, coherent composites. It would be very desirable to have a porous, microfiber reinforced composite for membrane filtration purposes, or to imbibe a second resin into the pores of the film to greatly enhance the conformability, adhesion to other materials and/or dielectric constant. Also, there is very little z-directionality to the LCP orientation in these teachings, even though it would be very desirable property to have.